The histocompatibility-2 (H-2) complex of the mouse consists of a series of closely linked loci on chromosome 17 which control protein and cell membrane antigens involved in a number of critical functions in cellular immunity. In recent years the demonstration of lymphocyte alloantigens which are closely linked but lie outside the traditionally defined H-2 complex, has prompted additional questions about the genetic organization of chromosome areas immediately adjacent to H-2. This proposal suggests three programs for expanding our knowledge of the genetic organization of the H-2 complex and for dissecting and defining the genetic control of the many important immunological phenomena controlled by genes located within or close to the H-2 complex. The first approach involves the development of a large number of intra-H-2 recombinants from a single heterozygote combination (H-2b/H-2t1) to provide two series of recombinant strains which are genetically identical except for a portion of the chromosome within the H-2 complex. The second approach involves an examination of crossing over outside the H-2 complex in the area immediately adjacent to H-2 with the intent of developing inbred strains which are identical at H-2 but differ with respect to small chromosome segments which lie near the complex. Finally, we will examine the genetic organization and immunological character of the S. region of the complex by studying the specificity of the Ss protein in different strains of mice and by screening for genetic recombination within the S region.